Can Blender import videos with transparent background?
Solved.
However I found some transparent format that Blender supports for transparency, it should be reencoded like this:
ffmpeg -i in.webm -vcodec prores_ks -pix_fmt yuva444p10le -profile:v 4444 out.mov

&quot;Blend&quot;-option should be set to &quot;Alpha Over&quot; in the N-panel of the strip.